What medication can I use for ADHD that is available in stores, w/o a prescription?

None . There are no effective medications for add available otc, that are more effective or safer than prescription medications. Why are you concerned? Your doctor needs to explain the dangers of otc treatments for add. Please see chadd.Org or nimh @ http://www.Nimh.Nih.Gov/health/publications/attention-deficit-hyperactivity-disorder/complete-index.Shtml.
None. There are no over the counter medications with evidence of benefit for adhd. There are some supplements that reportedly have helped some people but nothing with enough good studies that i could recommend them.
Caffeine, carefully. Caffeine, like in coffee, has helped some people -- but the dosage and response are not reliable. It would be better to get an evaluation with a psychiatrist who treats attention problems regularly. This may be someone who has done an additional fellowship in child and adolescent psychiatry. This physician could review your specific needs and prescribe more effectively for you.